First, even if this were true, what it would show is that there grew up around the historical hard core of the burial an invented motivation for why Joseph would have given Jesus an honorable burial, not that the burial of Jesus by Joseph of Arimathea was itself a legend since it is early, independently, and multiply attested; passes the criteria of embarrasment, etc. But what of the alleged legendary development in the Gospel narratives about Jospeh of Arimathea being a sympathizer and follower of Jesus?

1) In the later Gospels, we have independent attestation that Joseph was secretly a sympathizer of Jesus, (and John even says that he was disciple of Jesus), but secretly for fear of the Jews. Richard Bauckham has argued (I think anyway) that John is actually correct and that the reason Joseph's secret became more explicit in the Gospels after Mark has to do with protecting Joseph from the Jews and by the time of Luke and Matthew, and especially by the time of John, that it was no longer necessary to protect Joseph's affiliation with Jesus from the Jews because the members of the Sanhedrin that had condemned Jesus were either dispatched or dead because old age and/or the Jewish massacre.

2) Even in Mark's Gospel Joseph is described as a righteous man who was seeking for the kingdom of God which is the same kind of language that is used to describe the message taught by Jesus. This is important because this is not inconsistent or even radically different from the way Luke, Matthew, and even John speak of Joseph.

3) A sign of genuine history that has been called 'undesigned coincidences' by Tim McGrew has to do with the way in which independent narratives dovetail and interlock with one another in unintended ways. Interestingly enough, the Gospel of Luke interlocks with the other three because it mentions that Joseph wasn't present at the vote to condemn Jesus.

4) Other explanations serve to enhance the probability of the previous three points: Others have argued that Joseph was a delegate of the Sanhedrin to make sure that the body was dispatched properly; it was very important in Judaims that bodies be buried on the same day of execution so they were very solicitous about it. However, what should make us skeptical about this explanation is that it is very surprising that the none of the Gospels mention Jospeh taking care of the other two crucified men; it seems like they were content to let the Romans bury them in a common criminals' grave but Joseph singles out Jesus for selective treatment and buries him in the one of the most expensive tombs during that time which makes it very likely that Joseph really was a secret follower of Jesus.